It has been considered to apply nitride semiconductors to high-withstand-voltage, high-power semiconductor devices by utilizing their characteristics such as a high saturation electron velocity and a wide band gap. For example, GaN being a nitride semiconductor has a band gap of 3.4 eV, which is higher than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV), and has high breakdown electric field intensity. This makes GaN very promising as a material of semiconductor devices for power supply realizing a high voltage operation and a high power.
Many reports have been made on field-effect transistors, in particular, HEMT (High Electron Mobility Transistor) as devices using nitride semiconductors. For example, among GaN-based HEMT (GaN-HEMT), an AlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer has been drawing attention. In the AlGaN/GaN HEMT, a distortion ascribable to a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectric polarization caused by the distortion and spontaneous polarization of AlGaN, high-concentration two-dimensional electron gas (2 DEG) is obtained. Therefore, the AlGaN/GaN HEMT is expected as a high-efficiency switch element or a high withstand voltage, high power device for electric vehicles and the like.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2010-219247
In a nitride semiconductor device, a protection film is often formed through the deposition of an insulator covering a nitride semiconductor layer. In some case, what is called a MIS-type HEMT is formed with this protection film used as a gate insulating film. In the case where the protection film is formed, high-temperature annealing is applied to the protection film after its formation to improve its insulating film quality.
However, there has been found a problem that the high-temperature annealing, though improving the insulating film quality of the protection film, increases an off-leakage current in the nitride semiconductor device.
FIG. 1 is a characteristic chart illustrating a correlation of the off-leakage current with drain voltage in an AlGaN/GaN HEMT having a protection film. The protection film was formed by an ALD method (Atomic Layer Deposition method) with aluminum oxide used as a material. When a processing temperature is low (for example, 600° C.), there is almost no problem of the off-leakage current. On the other hand, when the processing temperature is high temperature (for example, 720° C.) at which the insulating film quality of the protection film significantly improves, it has been found out that the off-leakage current increases in accordance with an increase in the drain voltage.